Double-pulse LIBS is a promising technique for deep-sea applications.LIBS measurements in shallow water with up to 400 mJ each pulse were done to select laser parameters which promote optimized spectral line emission ...Double-pulse LIBS is a promising technique for deep-sea applications.LIBS measurements in shallow water with up to 400 mJ each pulse were done to select laser parameters which promote optimized spectral line emission from plasma even at elevated pressures,where line broadening until loss of most of the spectral information can occur.Optical emission spectroscopy,using a Czerny-Turner spectrometer,has been applied to investigate the dependence of the emitted radiation on laser parameters and hydrostatic pressure.It has been found,that higher laser pulse energies,especially with short pulse delay as required in high water pressure,can also have an adverse effect on the measured spectrum.展开更多
In this paper, we present a study on the effect of inter-pulse delay using femtosecond double-pulse laser-induced breakdown spectroscopy in a collinear geometry. The temporal evolution of spectral intensity is perform...In this paper, we present a study on the effect of inter-pulse delay using femtosecond double-pulse laser-induced breakdown spectroscopy in a collinear geometry. The temporal evolution of spectral intensity is performed for the lines of Fe I 423.60 nm, Fe I 425.08 nm and Fe I 427.18 nm. It is found that, by selecting appropriate inter-pulse delay, the signal enhancement can be significantly increased compared with the single-pulse case. A three-fold enhancement in the current experiment is obtained. The plasma temperature and electron density are also investigated based on the theory of Boltzmann plot and Stark broadening. We attribute the main mechanism for emission enhancement to the plasma re-heating effect.展开更多
We designed a femtosecond(fs)+picosecond(ps)double-pulse sequence by using a Mach-Zehnder-like apparatus to split a single 120 fs pulse into two sub-pulses,and one of them was stretched to a width of 2 ps by a four-pa...We designed a femtosecond(fs)+picosecond(ps)double-pulse sequence by using a Mach-Zehnder-like apparatus to split a single 120 fs pulse into two sub-pulses,and one of them was stretched to a width of 2 ps by a four-pass grating system.Through observing the ripples induced on the ZnO surface,we found the ionization rate appeared to be higher for the sequence in which the fs pulse arrived first.The electron rate equation was used to calculate changes of electron density distribution for the sequences with different delay times.We suggest that using a temporally shaped fs+ps pulse sequence can achieve nonlinear ionization control and influence the induced ripples.展开更多
In the photodetachment of atoms or negative ions by a double-pulse laser, the first pulse of the double-pulse laser generates waves and the delayed second pulse may detect them. The phenomenon of the excitation and de...In the photodetachment of atoms or negative ions by a double-pulse laser, the first pulse of the double-pulse laser generates waves and the delayed second pulse may detect them. The phenomenon of the excitation and detection of waves by a double-pulse laser can be used to identify the closed orbits in the system. We demonstrate this phenomenon with a negative hydrogen ion(H^-) by analyzing the total population excited by a double-pulse laser in a timedependent field for different physical parameters. By analyzing the total excited population using a double-pulse laser, we can uncover all the closed orbits existing in the system. We demonstrate that this can be realized by scanning the first pulse position and the time delay between the two pulses.展开更多
To make full use of expanded maneuverability and increased range,adaptive constrained on-board guidance technology is the key capability for a glide vehicle with a double-pulse rocket engine,especially under the requi...To make full use of expanded maneuverability and increased range,adaptive constrained on-board guidance technology is the key capability for a glide vehicle with a double-pulse rocket engine,especially under the requirements of desired target changing and on-line reconfigurable control and guidance.Based on the rapid footprint analysis,whether the new target is within the current footprint area is firstly judged.If not,the rocket engine ignites by the logic obtained from the analysis of optimal flight range by the method of hp-adaptive Gauss pseudospectral method(hp-GPM).Then,an on-board trajectory generation method based on powered quasi-equilibrium glide condition(QEGC)and linear quadratic regulator(LQR)method is used to guide the vehicle to the new target.The effectiveness of the guidance method consisted of powered on-board trajectory generation,LQR trajectory tracking,footprint calculation,and ignition time determination is indicated by some simulation examples.展开更多
Wide bandgap(WBG)semiconductors,such as silicon carbide(SiC)and gallium nitride(GaN),exhibit superior physical properties and demonstrate great potential for replacing conventional silicon(Si)semiconductors with WBG t...Wide bandgap(WBG)semiconductors,such as silicon carbide(SiC)and gallium nitride(GaN),exhibit superior physical properties and demonstrate great potential for replacing conventional silicon(Si)semiconductors with WBG technology,pushing the boundaries of power devices to handle higher blocking voltages,switching frequencies,output power levels,and operating temperatures.However,tradeoffs in switching performance and converter efficiency when substituting GaN devices for Si and SiC counterparts are not well-defined,especially in a cascode configuration.Additional research with further detailed investigation and analysis is necessitated for medium-voltage GaN devices in power converter applications.Therefore,the aim of this research is to experimentally investigate the impact of emerging 650/900 V cascode GaN devices on bidirectional dc-dc converters that are suitable for energy storage and distributed renewable energy systems.Dynamic characteristics of Si,SiC,and cascode GaN power devices are examined through the double-pulse test(DPT)circuit at different gate resistance values,device currents,and DC bus voltages.Furthermore,the switching behavior and energy loss as well as the rate of voltage and current changes over the time are studied and analyzed at various operating conditions.A 500 W experimental converter prototype is implemented to validate the benefits of cascode GaN devices on the converter operation and performance.Comprehensive analysis of the power losses and efficiency improvements for Si-based,SiC-based,and GaN-based converters are performed and evaluated as the switching frequency,working temperature,and output power level are in-creased.The experimental results reveal significant improvements in switching performance and energy efficiency from the emerging cascode GaN devices in the bidirectional converters.展开更多
The dispersive Fourier transform technique provides feasibility of exploring non-repetitive events and the buildup process in ultrafast lasers.In this paper,we report a new buildup process of dissipative solitons in a...The dispersive Fourier transform technique provides feasibility of exploring non-repetitive events and the buildup process in ultrafast lasers.In this paper,we report a new buildup process of dissipative solitons in a simplified mode-locked Yb-doped fiber laser,which includes more complex physics stages such as the Q-switching stage,raised and damped relaxation oscillation stages,noise-like stage,successive soliton explosions stage,and soliton breathing stage.Complete evolution dynamics of noise-like pulse and double pulse are also investigated with dispersive Fourier transform.For the noise-like pulse dynamics process,it will only experience the Q-switching and relaxation oscillation stages.In the case of dissipative soliton and noise-like pulse,the double pulse buildup behavior is manifested as the replication of individual pulses.A weak energy migration occurs between two pulses before reaching steady state.Meanwhile,real-time mutual conversion of the dissipative soliton and noise-like pulse has been experimentally observed,which appears to be instantaneous without extra physical processes.To the best of our knowledge,this is the first report on these physical phenomena observed together in a mode-locked fiber laser.The results further enrich the dynamics of mode-locked fiber lasers and provide potential conditions for obtaining intelligent mode-locked lasers with controllable output.展开更多
基金funded by the Deutsche Forschungsgemeinschaft(DFG,German Research Foundation)–Project No.454848899。
文摘Double-pulse LIBS is a promising technique for deep-sea applications.LIBS measurements in shallow water with up to 400 mJ each pulse were done to select laser parameters which promote optimized spectral line emission from plasma even at elevated pressures,where line broadening until loss of most of the spectral information can occur.Optical emission spectroscopy,using a Czerny-Turner spectrometer,has been applied to investigate the dependence of the emitted radiation on laser parameters and hydrostatic pressure.It has been found,that higher laser pulse energies,especially with short pulse delay as required in high water pressure,can also have an adverse effect on the measured spectrum.
基金supported by the National Basic Research Program of China(No.2013CB922200)the China Postdoctoral Science Foundation(No.2014M551169)National Natural Science Foundation of China(Nos.11674128,11474129 and 11504129)
文摘In this paper, we present a study on the effect of inter-pulse delay using femtosecond double-pulse laser-induced breakdown spectroscopy in a collinear geometry. The temporal evolution of spectral intensity is performed for the lines of Fe I 423.60 nm, Fe I 425.08 nm and Fe I 427.18 nm. It is found that, by selecting appropriate inter-pulse delay, the signal enhancement can be significantly increased compared with the single-pulse case. A three-fold enhancement in the current experiment is obtained. The plasma temperature and electron density are also investigated based on the theory of Boltzmann plot and Stark broadening. We attribute the main mechanism for emission enhancement to the plasma re-heating effect.
基金financially supported by the National Natural Science Foundation of China(Nos.12274280,11774220 and 11974147)。
文摘We designed a femtosecond(fs)+picosecond(ps)double-pulse sequence by using a Mach-Zehnder-like apparatus to split a single 120 fs pulse into two sub-pulses,and one of them was stretched to a width of 2 ps by a four-pass grating system.Through observing the ripples induced on the ZnO surface,we found the ionization rate appeared to be higher for the sequence in which the fs pulse arrived first.The electron rate equation was used to calculate changes of electron density distribution for the sequences with different delay times.We suggest that using a temporally shaped fs+ps pulse sequence can achieve nonlinear ionization control and influence the induced ripples.
基金the CAS-TWAS president fellowship program of UCAS for financial assistancepartial support by the National Natural Science Foundation of China (NSFC), grants No. 11 474 079 and No. 11 421 063。
文摘In the photodetachment of atoms or negative ions by a double-pulse laser, the first pulse of the double-pulse laser generates waves and the delayed second pulse may detect them. The phenomenon of the excitation and detection of waves by a double-pulse laser can be used to identify the closed orbits in the system. We demonstrate this phenomenon with a negative hydrogen ion(H^-) by analyzing the total population excited by a double-pulse laser in a timedependent field for different physical parameters. By analyzing the total excited population using a double-pulse laser, we can uncover all the closed orbits existing in the system. We demonstrate that this can be realized by scanning the first pulse position and the time delay between the two pulses.
基金supported by the National Natural Science Foundation of China(No.61403100)Fundamental Research Funds for the Central Universities(HIT.NSRIF.2015037)
文摘To make full use of expanded maneuverability and increased range,adaptive constrained on-board guidance technology is the key capability for a glide vehicle with a double-pulse rocket engine,especially under the requirements of desired target changing and on-line reconfigurable control and guidance.Based on the rapid footprint analysis,whether the new target is within the current footprint area is firstly judged.If not,the rocket engine ignites by the logic obtained from the analysis of optimal flight range by the method of hp-adaptive Gauss pseudospectral method(hp-GPM).Then,an on-board trajectory generation method based on powered quasi-equilibrium glide condition(QEGC)and linear quadratic regulator(LQR)method is used to guide the vehicle to the new target.The effectiveness of the guidance method consisted of powered on-board trajectory generation,LQR trajectory tracking,footprint calculation,and ignition time determination is indicated by some simulation examples.
文摘Wide bandgap(WBG)semiconductors,such as silicon carbide(SiC)and gallium nitride(GaN),exhibit superior physical properties and demonstrate great potential for replacing conventional silicon(Si)semiconductors with WBG technology,pushing the boundaries of power devices to handle higher blocking voltages,switching frequencies,output power levels,and operating temperatures.However,tradeoffs in switching performance and converter efficiency when substituting GaN devices for Si and SiC counterparts are not well-defined,especially in a cascode configuration.Additional research with further detailed investigation and analysis is necessitated for medium-voltage GaN devices in power converter applications.Therefore,the aim of this research is to experimentally investigate the impact of emerging 650/900 V cascode GaN devices on bidirectional dc-dc converters that are suitable for energy storage and distributed renewable energy systems.Dynamic characteristics of Si,SiC,and cascode GaN power devices are examined through the double-pulse test(DPT)circuit at different gate resistance values,device currents,and DC bus voltages.Furthermore,the switching behavior and energy loss as well as the rate of voltage and current changes over the time are studied and analyzed at various operating conditions.A 500 W experimental converter prototype is implemented to validate the benefits of cascode GaN devices on the converter operation and performance.Comprehensive analysis of the power losses and efficiency improvements for Si-based,SiC-based,and GaN-based converters are performed and evaluated as the switching frequency,working temperature,and output power level are in-creased.The experimental results reveal significant improvements in switching performance and energy efficiency from the emerging cascode GaN devices in the bidirectional converters.
基金This work was supported by the Beijing Natural Science Foundation(No.4192015)the National Natural Science Foundation of China(No.61975003).
文摘The dispersive Fourier transform technique provides feasibility of exploring non-repetitive events and the buildup process in ultrafast lasers.In this paper,we report a new buildup process of dissipative solitons in a simplified mode-locked Yb-doped fiber laser,which includes more complex physics stages such as the Q-switching stage,raised and damped relaxation oscillation stages,noise-like stage,successive soliton explosions stage,and soliton breathing stage.Complete evolution dynamics of noise-like pulse and double pulse are also investigated with dispersive Fourier transform.For the noise-like pulse dynamics process,it will only experience the Q-switching and relaxation oscillation stages.In the case of dissipative soliton and noise-like pulse,the double pulse buildup behavior is manifested as the replication of individual pulses.A weak energy migration occurs between two pulses before reaching steady state.Meanwhile,real-time mutual conversion of the dissipative soliton and noise-like pulse has been experimentally observed,which appears to be instantaneous without extra physical processes.To the best of our knowledge,this is the first report on these physical phenomena observed together in a mode-locked fiber laser.The results further enrich the dynamics of mode-locked fiber lasers and provide potential conditions for obtaining intelligent mode-locked lasers with controllable output.
